Data transmission using a block oriented digital transmission system such as orthogonal frequency division multiplexing (OFDM) or discrete multi-tone (DMT) signaling involves dividing a predetermined bandwidth into several narrowband channels, each channel having its own frequency of transmission. The data transmission in each narrowband channel can be affected by interference from external noise sources. OFDM and DMT are used in home networks were data is transmitted on pre-existing alternating current (AC) power lines. A power line is a noisy transmission medium for data communication. Noise sources, such as electronic and electro-mechanical sources that are generated by brush electric motors in home appliances, dimmer switches, fluorescent and halogen lights, etc., create impulse noise related to a 50 or 60 Hz power cycle. In addition, power supplies create harmonics associated with their switching frequency. Also, external transmissions, such as impulse noises and radio frequency (RF) interference from such sources as short wave and amateur radio, and other bands, can affect the quality of the channel on the power line. These noise sources interfere with reception of data signals, usually resulting in signal corruption, resulting in data errors, especially if the power of the external interfering signal is higher than the power of the transmitted OFDM signal. Information about channels affected by RF interference or other jammer signals is useful, and can be used to discard data sent on jammed channels. Several methods for detecting jammed channels using the analysis of signals received by a receiver are known in the prior art. A window function is one of the simplest means for analyzing the received signal.
For example, U.S. Pat. No. 6,111,919 issued to Yonge, III, on Aug. 29, 2000, titled “SYNCHRONIZATION OF OFDM SIGNALS” discloses a method for temporarily aligning a received symbol that is transmitted via OFDM channels, with a reference symbol and/or an earlier received symbol. According to Yonge, received information is processed by applying a window function, such as Hanning, a Hamming or a Blackman window. The window function is applied for filtering the signal prior to applying a fast Fourier transform (FFT) to the received signal. However, Yonge, does not use the window filter for identifying jammed channels, nor does he teach maintaining a list of jammed channels during OFDM data transmission.
Therefore, there exists a need for a simple method and apparatus that is adapted to identify jammed channels in an OFDM or DMT data transmission system, where an interference can corrupt the data transmission.